1. Field of the Invention
The present invention relates to a thermal image transfer recording medium comprising a support, and a thermofusible ink layer and a resin layer with a specific structure, successively formed on the support, capable of producing images with high density when it is used repeatedly.
2. Discussion of Background
Recording apparatus such as a printer and a facsimile apparatus to which the thermal image transfer recording method is applied have been widespread. This is because the recording apparatus of this type are relatively small in size and can be produced inexpensively, and their maintenance is simple.
In a conventional thermal image transfer recording medium for use with the thermal image transfer recording apparatus, a single thermofusible ink layer is merely formed on a support. When such a recording medium is used for printing images, those portions of the ink layer heated by a thermal head completely transfer to an image receiving sheet, such as a sheet of plain paper, at only one-time printing. Therefore, the recording medium can be used only once, and can never be used repeatedly. Such a conventional thermal image transfer recording medium is disadvantageous from the viewpoint of running cost.
There is an increasing demand for a thermal image transfer recording medium which can be used repeatedly, and many kinds of multi-printing type thermal image transfer recording media have been proposed as shown below:
(1) As disclosed in Japanese Laid-Open Patent Applications 54-68253, 55-105579 and 60-40293, there is proposed a thermal image transfer recording medium comprising a support and a microporous ink layer formed thereon, with a thermofusible ink impregnated in the microporous ink layer gradually oozing out. PA1 (2) As disclosed in Japanese Laid-Open Patent Application 58-212993, there is proposed a thermal image transfer recording medium comprising a support, and an ink layer and a porous layer successively provided on the support, with the amount of the ink which oozes out from the ink layer being controlled. PA1 (3) As disclosed in Japanese Laid-Open Patent Applications 60-127191 and 60-127192, there is proposed a thermal image transfer recording medium comprising a plurality of ink layers and a plurality of adhesive layers, each ink layer alternating with each adhesive layer. In this thermal image transfer recording medium, a pair of the ink layer and the adhesive layer can be gradually exfoliated as the images are printed, thereby always exposing an unused ink layer for the printing operation.
However, the above-mentioned thermal image transfer recording media have their own shortcomings.
For example, when the thermal image transfer recording medium (1) is used, the amount of the ink oozing out from the ink layer becomes insufficient after the repeated use of the recording medium. As a result, the image density of printed images gradually decreases as the number of printing times increases.
Regarding the thermal image transfer recording medium (2), the mechanical strength of the porous layer is decreased when the size of the pore included therein is increased to upgrade the image density, and thus the ink layer is apt to peel off the support together with the porous layer.
As for the thermal image transfer recording medium (3), the amount of the thermofusible ink transferred from the recording medium is not constant every time images are printed using the recording medium.
Furthermore, most of the conventional thermal image transfer recording media have been developed for a serial thermal head for use in a recording apparatus such as a word processor. Therefore, when those recording media are applied to a line thermal head for use in a recording apparatus such as a facsimile apparatus or a bar code printer, some troubles are produced, for instance, exfoliation of the ink layer, and decrease in image density of the printed images. This is because it takes long until the thermal image transfer recording medium is separated from the image receiving sheet from the time when thermal energy is applied to the thermal image transfer recording medium coming into contact with the image receiving sheet.
As disclosed in Japanese Laid-Open Patent Application 63-137891, the exfoliation of the ink layer from the support can be prevented by providing an intermediate adhesive layer comprising a heat-softening resin between the support and the ink layer in the thermal image transfer recording medium (1) or (2). In this case, however, the thermal loss is large during the thermal printing operation because of the provision of the intermediate adhesive layer. The thickness of the ink layer of the thermal image transfer recording medium capable of achieving the multi-printing operation is originally larger than that of the one-time thermal image transfer recording medium. Therefore the thermal sensitivity is remarkably decreased when the intermediate adhesive layer is provided in the multi-printing thermal image transfer recording medium. To overcome the above drawback, it is necessary to decrease the amount of the ink contained in the ink layer or increase the thermal energy applied to the recording medium.